When starting off on a road surface having different adhesive friction values between the right and left sides of the vehicle (μ-split), even when a relatively slight drive torque is applied, the wheel (low-μ wheel) located on the slippery side of the road surface begins to spin. If the speed of the low-μ wheel exceeds a specific slip threshold, the traction control system (TCS) intervenes in the operation of the vehicle and brakes the slipping wheel.
The braking torque exerted on the low-μ wheel is transferred via the differential to the wheel on the non-skid side of the road surface (high-μ wheel) and may be used there for the propulsion of the vehicle.
With a regulation of this type, the brake intervention on the slipping wheel converts the engine torque in the brake into heat. In vehicles with automatic transmission, this also has the consequence that the engine torque produced (less the torque acting in the brake) is converted into thermal energy in the torque converter of the automatic transmission, which may destroy the torque converter or the transmission even after a relatively short time.
During standing-start operations under μ-split conditions, situations may arise in particular with heavily loaded vehicles or vehicles with trailers in which the vehicle does not begin to move despite maximum drive torque, since the braking resistance torque and rolling resistance torque of the vehicle and the downgrade force acting on the vehicle and trailer are greater than the engine torque produced by the engine. In such situations, the result is an extreme load and a correspondingly rapid overheating of the torque converter.
Other traction control systems therefore include a converter or transmission protection function, which is implemented using a time counter, which is started in the control state “select high” when starting off on μ-split and below a vehicle speed of 5 km/h and which causes a forced switch into the state “select low” after a specified period of time (e.g., 15 seconds).
The state “select high” of a TCS is used to attain the maximum possible traction and is characterized by high slip thresholds for the drive wheels and a relatively high delivery of engine torque. In contrast, the state “select low” is used to attain the maximum possible vehicle stability and is characterized by slip thresholds set to be very sensitive and a correspondingly low engine torque.
The protective function of other systems provide that the TCS performs a rigid (time-controlled) switch independent of the actual load on the torque converter. Thus the speed of a vehicle moving slowly forward with slipping drive wheels may be limited just before reaching a non-skid road surface, although the temperature reached in the torque converter would not have required this yet.